Acute high-level exposures to chemicals that damage the respiratory tract can cause life-threatening lung injury. Chlorine gas is a highly toxic respiratory irritant that is considered a chemical threat agent because of the possibility that it could be released in industrial accidents or terrorist attacks. Chlorine, like many pulmonary toxicants, produces injury to epithelial cells lining the respiratory tract. Loss of the respiratory epithelial cell barrier contributes to other sequelae observed in acute lung injury, including pulmonary edema, airway obstruction, inflammation, and respiratory infections. Efficient repair of respiratory epithelium and restoration of barrier function are essential for resolution of chlorine-induced lung injury. The epithelium of the respiratory tract is repaired following injury through the proliferation and differentiation of stem cells. One type of cell involved in the repair of airway epithelium is the basal cell. The low-affinity nerve growth factor receptor (NGFR) has been identified as a marker for basal cells, as well as for epithelial stem cells in other tissues, but the role of this protein in stem cell function is unknown. NGFR binds multiple neurotrophin ligands, including nerve growth factor (NGF), brain-derived neurotrophic factor, and neurotrophin 3. The hypothesis to be tested in the proposed studies is that NGFR controls the proliferation, survival, and differentiation of lung epithelial stem cells following chlorine-induced lung injury. In Specific Aim 1, basal cells will be purified from the trachea of normal mice and from mutant mice lacking functional NGFR. The ability of the cultured cells to survive, proliferate, and differentiate into other types of epithelial cells (ciliated cells and Clara cells) will be measured in the presence or absence of NGFR ligands. In Specific Aim 2, wild-type, NGFR knockout, and NGF transgenic mice will be exposed to chlorine gas according to a protocol established in the Principal Investigator's laboratory. Effects of neurotrophins will be tested by delivering ligands intranasally or by using NGF-overexpressing transgenic mice previously developed in the Principal Investigator's laboratory. The extent of tracheal epithelial repair will be measured at various times after chlorine exposure. In Specific Aim 3, the effect of neurotrophins on epithelial repair following chlorine exposure in rabbits will be investigated. Rabbits will be used for these studies because of the wider distribution of basal cells in the airways in this species which is more similar to that in humans. The proposed experiments will not only investigate mechanisms of epithelial repair, but will also foster the development of therapeutic agents that could be used to stimulate resolution of lung injury following inhalation of toxic chemicals. PUBLIC HEALTH RELEVANCE: This application is submitted in response to PAR-10-019, CounterACT Exploratory/Developmental Projects in Translational Research (R21). The proposed experiments are designed to understand how the lung repairs itself after inhalation of a toxic gas with the goal of developing therapy for lung injury based on stimulating lungs cells to accelerate repair. This type of research is sought through the Funding Opportunity Announcement because of concerns that U. S. civilians could be adversely affected by highly toxic chemicals released intentionally in terrorist attacks or unintentionally in industrial accidents or natural disasters.